We have observed the opening and closing of single batrachotoxin-modified sodium channels in neuroblastoma cells using the patch-clamp method. The single-channel data together with voltage-clamp data on the same system could be accounted for by a 3-state closed-closed-open model, where a closed state refers to a molecular conformation of the channel that does not allow ions to pass and an open state refers to a molecular conformation of the channel that allows ions to pass with a conductance of about 10 pS. Transition rates between the states are exponential functions of membrane potential. One of the implications of this model is that there are always some closed channels, regardless of membrane potential.